Conventional vapor compression systems reject heat by condensation of the refrigerant at subcritical pressure given by the saturation pressure at the given temperature. When using a refrigerant with low critical temperature, for instance CO2, the pressure at heat rejection will be supercritical if the temperature of the heat sink is high, for instance higher than the critical temperature of the refrigerant, in order to obtain efficient operation of the system. The cycle of operation will then be transcritical, for instance as known from WO 90/07683.
WO 94/14016 and WO 97/27437 both describe a simple circuit for realizing such a system, in basis comprising a compressor, a heat rejector, an expansion means and an evaporator connected in a closed circuit. CO2 is the preferred refrigerant for these systems.
EP-A-10 043 550 relates to a compression refrigeration system using CO2 where an attempt is made to improve the heat pump efficiency of the system by controlling the compressor suction gas superheat.
Heat rejection at super critical pressures will lead to a refrigerant temperature glide. This can be applied to make efficient hot water supply systems, e.g. known from U.S. Pat. No. 6,370,896 B1.
Ambient air is a cheap heat source which is available almost everywhere. Using ambient air as a heat source, vapor compression systems often have a simple design which is cost efficient. However, at high ambient temperatures, the exit temperature of the compressor may become low, for instance around 70° C. for a trans-critical CO2 cycle. The desired temperature of tap water is often 60-90° C. The exit temperature of the compressor can be increased by increasing the exit pressure, but it will lead to a system performance drop. Another drawback with increasing pressure is that components will be more costly due to higher design pressures.
Another drawback occurring at high ambient temperatures is that superheating the compressor suction gas, which normally is provided by an internal heat exchanger (IHX), is not possible, as long as evaporation temperature is higher than the heat rejector refrigerant outlet temperature. Hence, there is a risk of liquid entering the compressor.
A strategy to solve these problems is to regulate the evaporation temperature such that it is below the heat rejector refrigerant outlet temperature. This will make superheating the suction gas possible and also increase the compressor discharge temperature for better hot water production; however, the system energy efficiency will be poor since suction pressure will be lower than necessary.
U.S. Pat. No. 6,370,896 B1 presents a solution to these problems, by using a part of the heat rejector to heat the compressor suction gas. The full flow on the high pressure side is heat exchanged with the full flow on the low pressure side. This will ensure superheating of compressor suction gas, and thereby secure safe compressor operation; however, the system efficiency drops compared to a system which compresses saturated gas (if possible) and which operates with a higher exit pressure to achieve a sufficient compressor discharge temperature.